1. Field of the Invention
The invention is directed to an improved ink composition for ball pens, the ink being of the type whcih produces a mark on a writing surface of initially erasable character by means of an ordinary pencil eraser but which becomes permanent after a period of time of a few hours. Inks of this type are well known as are illustrated, for example, by Daugherty et al. U.S. Pat. No. 3,875,105; Muller et al. U.S. Pat. No. 4,097,290; Muller U.S. Pat. Nos. 4,329,262 and 4,329,264; and Ferguson U.S. Pat. No. 4,390,646.
2. Description of the Prior Art
The prior art suggests various elastomers in a mixed solvent system together with colorant to form a pressurized pen ink composition. The Ferguson '646 patent suggests the use of block copolymers having butadiene or isoprene midblocks which may replace the use of natural rubber as in the Daugherty et al. '105 patent to eliminate the necessity for milling the rubber. The Muller '264 patent suggests the use of a solvent system containing little or no volatile component. The Muller et al. '290 patent suggests the use of a mixed solvent system emphasizing low boiling and high boiling components. The Muller '262 patent suggests the use of a mixed solvent system comprising a volatile component and an essentially non-volatile component having a low viscosity and the optional additon of non-volatile, high viscosity material.